The turnover of membrane inositol-containing phospholipids are closely linked to cellular responses to hormones and neurotransmitters. The factors controlling the action of the enzymes catalyzing the formation and degradation of inositol-containing phospholipids should regulate the membrane functions associated with those phospholipids. This project proposes to study the action of a homogeneous preparation of the membrane-associated phosphatidylinositol synthase in well defined systems. The phosphatidylinositol synthase will be purified from Saccharomyces cerevisiae by affinity chromatography techniques based on substrate affinity and substrate elution using CDP-diacylglycerol. The enzymological properties of phosphatidylinositol synthase will be initially studied using detergent-phospholipid mixed micelle systems. Liposome systems in the absence of detergent will then be developed for studying the factors that control phosphatidylinositol synthase activity in an environment that more closely mimics in vivo conditions. The enzymological properties that will be studied include the specificity of the enzyme for CDP-diacylglycerol consisting of a variety of fatty acid moieties, and the action of the enzyme in the presence of hormones and neurotransmitters. The information obtained from these studies will be used to develop a model for the regulation of phosphatidylinositol synthesis and membrane mediated responses.